We are making a comparative study of behavioral reactions to two classes of stress; namely, the internal malaise induced by toxins and the external insult of electric shock. Signal effectiveness, temporal parameters as well as the neural substrate for conditioning and learning in these two spheres differ markedly. For example, when external shock is used as an immediate reinforcer, auditory or visual signals are effective for avoidance learning while a flavor is not. On the other hand, when internal toxicosis is used as a reinforcer a flavor signal is effective for hours following tasting, and external signals are useless for avoidance training. In addition, we have made brain lesions which disrupt noise-shock conditioning while enhancing flavor-toxicosis learning. Thus, animals are specialized or biased to form some associations and not others, and this bias must be inherent in the structure of the afferent channels and the integrativebrain mechanisms which handle the particular signals and the specific reinforcers used in conditioning. We are attempting to specify this organismic bias with arrays of behavioral tests and to relate this bias to central convergence of the afferent channels, using brain stimulation and lesion techniques.